In a machine, such as an excavator, a backhoe or a shovel, a hydraulic circuit may include a variable displacement pump in fluid communication with one or more hydraulic actuators to handle a variable load. The pump provides pressurized hydraulic fluid to each of the actuators, such as a hydraulic cylinder or a hydraulic motor, to move the load. The actuators may be connected to work implements, such as a boom, stick, bucket and/or swing gear train.
A typical digging operation for an excavator or other machine with an implement, may have a plurality of phases. Such phases may include, but are not limited to, an initial phase, a digging phase, a digging-boom-up-overrunning-load phase, a digging-boom-up-light-resistive-load phase and a boom-lift phase. In the initial phase, there is no digging load and the boom, stick and bucket are moved into position to begin digging. In the digging phase, the boom is generally held in place while an implement, for example a stick and bucket, attached to the boom digs. In the digging-boom-up-overrunning-load phase, the boom is moved upward while the implement is digging. In such phase, the reaction digging force applied on the boom cylinder through the implement is greater than the resistive force of gravity. In the digging-boom-up-light-resistive-load phase, the boom is moved upward while the implement is digging but the reaction digging force is less than the resistive force of gravity. In the boom-lift phase, the implement is no longer digging and the boom is moved upward along with the load contained in the implement.
When the hydraulic circuit transitions between the digging phase to the digging-boom-up-overrunning-load phase, the boom portion of the hydraulic circuit generally transitions from a holding operation to a lifting operation, the bucket and stick circuits carry a high digging load, and the pump must supply fluid at high pressure to support the digging function. As a result, for a short period of time, for example, about 0.5 to about 2 seconds, the boom cylinder may be in an overrunning load condition. When lifting the boom with an overrunning load condition, the head end of the actuator for the boom receives pump flow with a greater pressure than necessary which, consequently, may cause pressure modulation by a compensation valve disposed downstream of the pump and upstream of the head end of the actuator. A relatively large amount of power may be dissipated due to the fluid pressure drop across the compensation valve. Similar power dissipation may occur when the hydraulic circuit transitions to the digging-boom-up-light-resistive-load phase. This power dissipation could be reduced.
JP2012-172491 discloses a hydraulic system that includes a flow volume limiting means which limits the flow volume supplied to the head side of a boom hydraulic cylinder from a hydraulic pump. Such restriction occurs only at the time of a boom raising operation where the rod lateral-pressure force of the boom hydraulic cylinder is higher than the head lateral-pressure force. A better system is desired for conserving energy in a hydraulic system.